So I think I found the issue. The page that aternos uses for mythicmobs hasn't updated to 5.0.4 and above. I've contacted Mythicmobs a few days ago but they haven't responded.
Maybe if multiple people brought this up they might look into it sooner.
Another option, is using the spigot page instead of the Curseforge one as that page gets updated more frequently. However this option would require extra work from the aternos team.